Pipex loop
We had a sales guy who had his email on Pipex.
One day his office based subordinate was going on holiday, and set their email to forward to him.
All went well till a virus infected email our system didn't detect came in. It forward it to pipex, who rejected it due to the content.
The NDR was duly forwarded, and rejected.
Cue 1 email loop around and around, slowly getting larger and larger.
It was about 3meg when we found it, on the n-hundredth loop.
It could been worse, but the slowness of our single ISDN channel kept the loop from going any faster!